The corpus luteum (CL) has a critical role in reproductive physiology due to secretion of progesterone, a hormonal requirement for pregnancy in mammals. However, if pregnancy does not occur, CL undergo an intriguing process termed luteolysis that is characterized by decreased progesterone production and death of cells in the CL. This research explores the in vivo mechanisms of luteolysis. The intracellular signaling pathways and gene expression cascades associated with protection or sensitization to luteolysis will be defined. First, an analysis will be done of the transcriptosome (steady state mRNA concentrations) in the CL using bovine microarray analysis of about 20,000 different mRNA transcripts. A more systematic analysis will also be performed of the changes in the transcriptosome that are induced by the hormone causing luteolysis, prostaglandin F2alpha (PGF), in CL that have the ability to undergo luteolysis in response to PGF (luteolytic capacity) or in CL without luteolytic capacity. Second, an in vivo model will be validated that produces CL with a large fluid-filled cavity allowing intraluteal treatments and monitoring. This model is central to our future studies of luteolysis because, despite considerable effort, no in vitro system has been developed that fully mimics in vivo luteolysis making it difficult to validly explore intracellular signal transduction during luteolysis. This in vivo model will be used to explore 2 key intracellular pathways that may be central to luteolytic sensitivity. Novel hypotheses will be explored on the role of constitutively active protein kinase A (PKA) in high constitutive progesterone production and the changes in PKA during luteolysis. We will also examine the luteal responses "protected" from PGF action by high intraluteal progesterone and the role of this pathway in luteolytic sensitivity. SPECIFIC OBJECTIVE 1: Characterize luteolysis-induced changes in the transcriptosome of the CL. This Objective will use bovine microarrays and differential display to determine the changes in mRNA that are induced by PGF in CL with and without luteolytic capacity at 2 times after PGF treatment (1 h and 10 h). SPECIFIC OBJECTIVE 2: Characterize an in vivo model for intraluteal treatment of bovine CL. SPECIFIC OBJECTIVE 3: Determine the roles of PKA in luteal function and luteolytic capacity. Studies will explore how PKA may be involved in high luteal progesterone and luteolytic capacity. SPECIFIC OBJECTIVE 4: Determine the role of intraluteal progesterone in luteolytic capacity. Completion of this research will validate a new in vivo model for luteolysis and provide insight into the interactions of specific gene expression cascades and signal transduction pathways during luteolysis.